Most of national and international standards for AV determination are based on the acid-base titration techniques in the non-aqueous systems (ISO 660-1983 (E); ASTM D664-89; AOCS Ca 5a-40-1989; GOST 10858-77, 5476-80, USSR). These techniques are time--and labour consuming, difficult for automation, particulary on the industrial line. They use toxic solvents, for example: diethylether, methyl-isobutyl ketone and others, as well as non-aqueous titrants which are very sensitive to carbon dioxide contamination from the atmosphere.
A number of techniques for AV determination without titration have been suggested. There is the pH-metric technique in the presence of a special reagent [T.M. Lapshina et al., Zh. Anal. Khim. 46(1991), 1150], chromatographic technique [R. G. Ackman, Food Sci. Technol., N.Y., 53 (1992) 47] as well as the spectroscopic technique with solvent [A. A. Ismail et al., J. Am. Oil Chem. Soc., 70 (1993) 335] and without solvent (T. K. Blumenthal et al., Am. Oil Chemists' Soc. 84th Annual Meeting and Exposition, Anahein, Calif., Apr. 28, 1993).
The techniques without titration mentioned above are not free of certain drawbacks as well. They use toxic solvents (pH-metry, chromatography, spectroscopy with solvent), use expensive and complicated instruments (chromatography and spectroscopy). The spectroscopic technique, without solvent, is time-consuming and difficult for automation.
The technique--precursor for our invention (T. M. Lapshina et al., Zh. Anal. Khem., 46(1991), 1150) was based on the reagent (0.15-0.20 mol/L triethanolamin (TEA) in the solvent 80% diethylether+19% C.sub.2 H.sub.5 OH+1% H.sub.2 O, % Vol.) and pH-metry. An oil sample was completely dissolved in the reagent. At invariable ionic strength and large excess of TEA in comparison with the sum of the determined acids a linear dependence was observed: EQU pH'=const.-1 g N.sub.a, (1)
where pH' is conditional pH of the oil solution in the reagent. The conditional character of pH value is caused by the use of aqueous buffer solutions for pH-meter calibration; all pH values given below are conditional ones (pH'). The value "const." in eqn(1) is the constant value for the given ionic strength, reagent concentration and pH-sensor; N.sub.a is the number of equivalents of the determined acids per litre of the reactive mixture.
The AV value was determined on the basis of eqn(1) by the standard addition method at the condition that the standard acid was added to the reagent after the oil sample: ##EQU1## where 56.11 is molecular weight of KOH; N.sub.st and V.sub.st are the concentration (eq/L) and the volume (mL) of the standard acid addition respectively; m is the weight of the oil sample (g); .DELTA.pH'=pH'.sub.1 -pH'.sub.2 ; pH'.sub.1 is pH of the oil solution in reagent; pH'.sub.2 is pH of the previous solution after addition of the standard acid.
The drawbacks of the technique-precursor (see T. M. Lapshina et al., Zh. Anal. Khim., 46 (1991) 1150) are the following:
A) The reagent is toxic.
B) Low water concentration in the reagent causes insufficient stability in work of the pH-metric sensor with glass indicator electrode and standard aqueous reference electrode.
C) The non-aqueous base reagent is very sensitive to carbon dioxide contamination from the atmosphere because of the insoluble carbonate formation which may change the pH value of the reagent (pH'.sub.0).
D) The acid solution being used as standard addition (stearic acid in chloroform) is toxic and its standardization is difficult.
E) The choice of the optimal N.sub.st and V.sub.st values depends on unknown AV value that may require repeated addition or reanalysis which prolongs the analysis.